1. Field of the Invention
The present invention relates to stave coolers used for cooling, for example, hearth walls of blast furnaces, etc.
2. Description of the Prior Arts
Generally speaking, the life of a blast furnace using stave coolers is said to depend on the durability of the staves.
However, up-to-date the staves are made of low-melting point, fragile cast iron and thus are quite susceptible to severe damages due to fusion, thermal crackings, high-temperature wearing and the like. The wear of staves is caused by a peculiar phenomenon that the graphite flake (kish graphite) in the stave cast iron is attacked by CO.sub.2, SO.sub.2, K.sub.2 O, etc. contained in the furnace gas to form a plate-like, fragile defect portion attacked like an ant nest in the stave cast iron, so that the stave is easily damaged due to wearing or cracking caused by the furnace charges.
Therefore, in order to reduce the wearing rate of the staves, it is essential that a heat resistant cast steel free from the graphite is used as the stave material.
However, there is another problem that the melting point of the cast steel is remarkably different from that of the cast iron. The cast iron can be easily cast at a relatively low temperature ranging from 1300.degree. C. to 1350.degree. C. so that the cast iron stave is substantially free from the problem of fusion of pipes embedded in the staves and the pipes can be consistently and safely cast in the staves. On the other hand, the cast steel must be cast at about 1550.degree. C., and requires risers (sink heads) for preventing shrinkage during the casting operation. The portion provided with the riser is delayed in solidification so that the fusion of the cooling pipes in this portion is caused.
In order to prevent the fusion of the cooling pipes, it is necessary to increase the thickness of coatings applied on the cooling pipes. This not only leads to remarkable lowering of cooling capacity of resultant cooling staves, but also increases the tendency of stripping-off of the coatings due to the thermal shocks during the casting operation and considerably promotes the pipe fusion so that the staves can not be consistently produced.
For these reasons, a stave made of cast steel has never been practically made or used in the blast furnace.